In the continuous casting, the molten steel is poured into the tundish through a ladle, and undergoes a conspicuous temperature drop due to the heat dissipation from the poured flow, the heat absorption by the inner lining refractory material, and heat radiation from the surface of the bath.
As the ordinary technique which compensates such a temperature drop, there has been a technique disclosed in Japanese Patent Laid-Open No. 163,730/79 in which a vertical type induction heater adapted to vertically circulate the molten metal is attached for heating to the bottom wall of the molten metal storing container. However, since in the technique disclosed therein, the vertical type induction heater is used in the attached state to the bottom wall, it is difficult to use in a tundish for the continuous casting apparatus.
On the other hand, there has been heretofore proposed a technique by which a horizontal channel type induction heater is fitted to the side wall of the tundish, as disclosed in Japanese Patent Laid-Open No. 56,144/82. A skeleton view of the heater used in this technique is shown in FIGS. 1 and 2. The illustrated horizontal channel type induction heater 2 is fitted to the side wall of the tundish 1. The body of the induction heater 2 is constitued by disposing a refractory material 7 inside of a shell 6 defining the outer shell, and has a roundabout or circular channel 8 formed in a loop shape from the inlet port 8a to an outlet port 8b which are opened to the interior of the tundish 1 and a through hole 9 provided penetrating the central portion surrounded by the roundabout or circular channel 8 in a direction orthogonal to the flowing direction of the molten steel. In FIGS. 1 and 2, a reference numeral 3 denotes the location of a nozzle from which the molten steel is received, a reference numeral 4 an outflow port, and a reference numeral 5 a partition wall for guiding the molten steel flow, which is provided if necessary.
A primary induction coil 10 to generate an induction current i in the molten steel flow within the roundabout or circular channel 8 is assembled through insertion in the inside of the above through hole 9 via a core 10a. A magnetic field .phi. is produced in the core 10a when the primary induction coil 10 is energized, and the secondary induction current i is accordingly flown in the molten steel within the roundabout channel 8, so that a Joule's heat of i.sup.2 .multidot.R is produced to heat the molten metal. To put it into another words, the heater is so constituted that the molten steel passage as the roundabout channel 8 is provided to heat the molten steel during the roundabout or circular movement.
However, when this induction heater 2 is used, there have been often experienced that the intended heating of the molten steel may not be appropriately and smoothly performed depending upon the schedule of the power supply to the heater.
That is, when a normal rated electric power is constantly supplied to the coil 10 of the induction heater 2 in the heating of the molten steel, since air is often stayed in the roundabout channel 8 particularly in case that the stored amount of molten steel in the tundish is small, that is, in the initial stage of pouring the molten steel from the ladle to the tundish 1, at which heating is most necessary, the sectional area of the molten steel flow becomes smaller and the secondary induction current density becomes larger in the roundabout channel 8 in which the air is stayed, so that the pinching phenomenon in the roundabout channel 8 becomes conspicuous and in the worst case, the molten steel is cut off in the channel 8 to interrupt the induction current. When the pinching phenomenon becomes conspicuous like this, fluctuation in the electric current flowing through the coil 10 becomes larger so that the electric power necessary for heating the molten steel can not be steadly supplied, and in some cases, there takes place a tripping of the electirc power source. In the case of the electric continuity interruption due to the pinching phenomenon, it takes a long time to recover, and similar electric continuity interruption repeatedly comes to occur. When the above pinching phenomenon becomes more conspicuous, the damage of the refactory material layer is too large to be repaired, and there comes out a possible molten steel leakage. Although restriction of the electric power to be supplied is effective for prevent such a phenomenon, the temperature drop of the molten steel aimed at in the initial stage can not be avoided.
The presence or lacking of the pinching phenomenon accompanied by the properness or improperness of the electric power supply schedule comes into almost no problem in the case of the ordinary vessel for holding the molten metal other than the tundish for the continuous casting. For, in the case of such a holding vessel, it is not late to apply an electric power after the bath surface level is so raised that the electric power may be stably supplied. However, when the temperature drop is large in the pouring initial state in the case of the tundish for the continuous casting, the casting sheet quality is adversely affected. Thus, the control of the electric power supply is indispensable for meeting the requirement that the application of the electric power is harstened at the early stage to facilitate heating.
In this sense, it is necessary to develop technique to effectively prevent the temperature drop by supplying the maximum electric power within a range in which no pinching is caused and at the initial stage of pouring the molten metal into the tundish. It is just an object of the present invention to provide a method of heating the molten steel in the tundish by using horizontal channel type induction heater which meets such a requirement.